Fake Motion Blur on Video Footage using Compositor
I am afraid that there is no optical flow available in Blender at this time. Optical flow technology uses motion vectors to estimate motion vectors across an image.

To simulate something similar try animating a pseudo mask object that follows the moving 2D image. You can animate it manually or you could use the tracker to follow elements of the animation then hook an object to it in 3D view. Take these actual object movements to use as vectors for the compositor.

For a better result, you can separate the character from the background if you have a clean plate of the background using a Difference Key node.